1. Field of the Invention
The invention relates to isolation means particularly those employed on bidirectional telephone or communication lines for isolating the transmitted signal from the received signal.
2. Prior Art
In recent years, because of judicial and regulatory decisions, many companies are now manufacturing equipment which connects directly to the public telephone network in the United States. The equipment so connected must meet certain specifications, for example, those involving isolation and impedance matching.
One of these requirements which is particularly significant for modulator/demodulator units (modems) is that the telephone line must be terminated in a constant impedance. Typically, in the United States, this impedance is 600 ohms resistive with little or no reactive components. This impedance is, at least in theory, equal to the line impedance. However, in practice, the line impedance is seldom 600 ohms and often contains a reactive component (capacitive or inductive) which varies between 100 ohms to 1.2k ohms.
A duplexer is generally used to separate or isolate the transmitted signal from the signal received on the bidirectional telephone lines. In FIG. 1, a typical duplexer is shown within the dotted line 46 and will be discussed along with the improvement to the duplexer provided by the present invention.
In modems and other devices, transmission and reception frequently occur at the same time when they are used in full duplex mode. Since the transmitted signal is generally substantially larger than the received signal, circuitry must be provided to prevent the transmitted signal from being detected as the received signal or interfering with the proper detection of the received signal. By way of example, 300 baud modems in the answer mode transmit at 2025 Hz to 2225 Hz and receive at 1070 Hz to 1270 Hz. In the originating mode, they transmit at 1070 Hz to 1270 Hz and receive at 2025 Hz to 2225 Hz. These frequency bands are relatively close, making it difficult to provide complete isolation. When the line impedance is 600 ohms, and the duplexer terminates the line in 600 ohms, the duplexer provides satisfactory isolation in conjunction with reasonable bandpass filters. However, in practice, because of the variations in line impedance, a duplexer may only provide 10 db or less of isolation, thus increasing the amount of filtering required as described below.
Filters are typically used to provide further isolation. These filters limit the bandwidth of the transmitted frequency, and likewise, limit the bandwidth of the received signal. However, unless quite expensive filters are used, the "skirts" of these filters overlap, thus in some cases some of the transmitted signal passes through the filters (and duplexer) to the receiver where it may be detected as a received signal or interfere with the proper detection of the received signal.
The problem of compensating for telephone line impedance fluctuations is an old problem, and numerous circuits have been proposed to compensate for such impedance variations. The prior art known to Applicant does not provide the terminating of the telephone line in a constant impedance. In some cases, a conjugate match to the line is made; obviously in these cases the line is not terminated in a pure resistive load (e.g., 600 ohms). The prior art known to Applicant is U.S. Pat. Nos. 4,103,118; 4,096,362; 3,982,080; and 3,178,521. Other related art which uses a passive network and manual balancing is described in U.S. Pat. Nos. 3,496,292 and 2,186,006.
As will be seen, the present invention provides an improved duplexer which includes a circuit for compensating for line impedance variations. The line is terminated in a pure resistive load with the described invention. With the use of the present invention less expensive filters may be employed, by way of example, in modems, since better isolation is obtained from the duplexer.